1. Field of the Invention
The present invention is directed to an improved system for post-detection processing of information in radar receivers.
2. State of the Prior Art
In medium PRF radar systems employing multiple PRF's, "ghost" or "false" target indications often appear where very strong "discrete" sidelobe clutter return signals correlate with true target return signals occurring in corresponding ambiguous range gates of the multiple PRF's. In the prior art, the problem of distinguishing true target returns from discrete clutter returns in corresponding ambiguous range gates has been approached with significantly diverse systems.
Generally, a medium PRF pulse doppler (PD) type radar is characterized by having an ambiguous range measurement, for each received pulse. By contrast, in a low PRF system, the time period between each transmitted pulse allows time for the transmitted pulse to be reflected from a within-range target and to be received by the receiver before the next pulse is transmitted. Therefore, in a low PRF system, the range of each return signal is unambiguous since the time between the transmitted pulse and the corresponding received signal is known. However, in a medium PRF system, the transmitter transmits a pulse, and prior to the pulse having time to be returned by a within-range target, subsequent pulses are transmitted. The interpulse period (time between each transmitted pulse) in the medium PRF system is shorter than that for the low PRF system. As a result, pulses received by the radar receiver in a medium PRF system are ambiguous in range, since the return signal may occur in one of a plurality of interpulse periods subsequent to the period immediately following the corresponding transmitted pulse.
In medium PRF radar systems, each transmitted pulse is coherent with respect to every other transmitted pulse, and the Pulse Repetition Frequency is high enough to monitor high or low speed doppler shifts in the return signals. Used in a high speed aircraft environment, such doppler shift information may be used to determine the speed, acceleration and direction of motion of a moving target. The presence of doppler shift in the return signals also provides a basis for distinguishing true targets from main beam (stationary) clutter signals returned through the main beam of a directional antenna. However, since the true target returns occur in ambiguous range gates, it is quite possible and highly probable that close-in return signals from discrete ground clutter may be picked up in the sidelobes of the directional antenna and will occur in corresponding ambiguous range cells. Subsequent resolving of the inherent ambiguous range into the unambiguous range for each signal will result in many false targets appearing at widely different ranges.
In a typical prior art radar receiver, as shown in FIG. 1A, also discussed in commonly assigned U.S. application Ser. No. 665,348, entitled TARGET DETECTION SYSTEM IN A MEDIUM PRF PULSE DOPPLER SEARCH/TRACK RADAR RECEIVER, two separate receiver channels, 10 and 20 are used in order to distinguish true target return signals from main beam clutter and discrete sidelobe clutter in a medium PRF system. A directional main antenna 1 is connected through a duplexer 5 to the main channel coherent receiver 7. The directional main antenna 1 is characterized by having a high gain directional main beam reception response and a low gain sidelobe reception response, as is comparatively shown in FIG. 1B. A separate guard receiver channel 20 includes a guard antenna 2 connected to a coherent receiver 8. The guard antenna is separate from the main antenna and, as shown in FIG. 1B, has a characteristic broad beam gain response which is comparatively higher than any sidelobe of the main antenna 1 and is comparatively lower than the main beam of the main antenna 1. The guard channel 20 will necessarily receive the same signal information as is received in the main channel 10. However, due to the reception characteristics of the respective main antenna, signals received in the main beam of the main antenna 1 are output with a higher amplitude than are corresponding signals received in the guard antenna 2. Correspondingly, return signals received in the main antenna sidelobes are output with a lower amplitude than the corresponding signals received by the guard antenna. Typically, the main channel 10 and the guard channel 20 contain filter banks 9 and 12, respectively. These filter banks are effective in acting as main beam clutter rejectors by filtering the output of the corresponding coherent receiver 7 and 8, and rejecting signals which are doppler shifted by an amount corresponding to the stationary ground targets in the main beam (main beam clutter) with respect to the speed of the radar-carrying aircraft. Since the main beam clutter has a predictable doppler shift, determined by the speed of the radar-carrying aircraft with respect to the stationary targets in the main beam, only signals which are doppler shifted away from the main beam clutter frequency are passed through the filter banks in each channel. The signals which are passed by the filter banks in each channel, may include both area and discrete sidelobe return signals, main beam target return signals from targets moving with respect to main beam clutter and noise, since both discrete sidelobe return signals and moving target return signals in the main beam have dopplers which are shifted away from the predicted doppler shift of the main beam clutter.
The signals passed by the filter bank 9 in the main channel 10 are fed into a conventional constant false alarm rate (CFAR) threshold circuit 11. The CFAR threshold circuit has an adaptive threshold level which raises or lowers to block out area sidelobe clutter, but is ineffective against strong discrete sidelobe clutter signals.
As discussed previously, due to the antenna characteristics, the guard channel 20 receives the sidelobe clutter with a higher amplitude than does the main channel 10. On that principal, the prior art embodiment shown in FIG. 1A, operates so that the detection of the sidelobe clutter in the guard channel 20 is used to cancel or blank out the correspondingly detected clutter which occurs in the sidelobes of the main antenna 1 and is amplified in the main channel 10.
The output of the guard channel 20 is fed into the amplitude comparison and blanking circuit 13, wherein the amplitudes of the discrete doppler shifted signals, corresponding in frequency, are compared. Where the amplitude of a discrete frequency guard channel signal exceeds the corresponding signal amplitude in the main channel, that discrete frequency is blanked from further processing. Since the guard channel 20 normally receives the true target returns at an amplitude far reduced than that of the main channel, the effect of the guard channel is to cause cancellation of only the discrete sidelobe clutter returns without effecting the true target returns received in the main channel 10.
Following the blanking operation, the range ambiguity resolver 15 receives the true target signals, which are ambiguous in range, and resolves the range. If a plurality of PRF's are transmitted, the receiver 15 divides the PRF's into a predetermined number of range cells and correlates the reception in corresponding range cells of each PRF. The signals which occupy corresponding range cells in each of the other PRF's are correlated, and the true target return signal is determined when it appears in a range cell common to each of a predetermined number of PRF's. The principles of this technique are described, for example, in THE RADAR HANDBOOK, McGraw-Hill 1970, pages 19-13 through 19-17, as well as "Multiple High PRF Ranging" by W. A. Skillman and D. H. Mooney, Proceedings of the 1960 IRE Conference on Military Electronics, pages 37-40. The above prior art system thereby resolves the range ambiguity and distinguishes true targets from sidelobe clutter. The target is then displayed in a conventional manner on a PPI display, according to the resolved unambiguous range measurement.
Problems with the prior art guard channel receiver system as described above, and shown in FIGS. 1A and 1B, are manifest, due to the redundancy necessitated by the guard channel circuitry and guard channel antenna mechanism. Such redundancy is detrimental, since it is expensive and also adds additional weight to the aircraft.
Attempts to eliminate the above technique have produced other prior art attempts to eliminate sidelobe discrete clutter. One of those techniques involves the use of an ultra-low sidelobe antenna so that sidelobe clutter discrete returns are not detected, to thereby reduce the occurrence of "ghost" target indications. Such an ultra-low sidelobe antenna used in a radar system on a modern high speed aircraft, appears to work well at higher altitudes, but is not effective in "on-the-deck" operations where discrete sidelobe clutter returns are high and are necessarily received, due to the close proximity of the aircraft to the discrete clutter targets.
Another prior art attempt at solving the problems described in connection with the guard channel radar receiver as typically described with respect to FIGS. 1A and 1B is the single channel radar receiver post detection-STC system shown in FIG. 2. The prior art system in FIG. 2 has the advantage of a single channel receiver, thereby eliminating the guard channel and the corresponding redundant hardware necessitated thereby. The prior art system shown in FIG. 2 also eliminates the costly use of the relatively expensive ultra-low sidelobe antenna, since it may be used with a more conventional directional antenna similar to that shown in FIG. 1 as main antenna 1. FIG. 2 shows a post detection system which operates in conjunction with a receiver similar to that shown as the main channel 10 in FIG. 1, wherein the received signals from the coherent receiver are received by a conventional doppler filter bank 24. The doppler filter bank 25 rejects those signals which are doppler shifted by a predetermined amount corresponding to main beam ground clutter. The remaining signals are passed by the doppler filter bank 25 to a conventional CFAR threshold circuit 27. As described previously, the CFAR threshold circuit typically has an adaptive threshold level which raises or lowers to block area sidelobe clutter return signals, but is ineffective against strong discrete sidelobe clutter return signals. The use of a filter bank and a CFAR threshold circuit are well known in the art, and are typically described in U.S. Pat. No. 3,701,149.
The post detection-STC circuit shown in FIG. 2 further includes a range unfolder 29, which receives the detected data from the CFAR threshold circuit indicating received signals and their corresponding amplitudes, for each PRF. The range gate corresponding to each detected data (ambiguous) is unfolded into true range cells (unambiguous). Therefore, for example, a detection in ambiguous range gate No. 3 in a first PRF having a total of 50 gates between each transmitted pulse will result in the range unfolder circuit 29 placing the detection in cells 3, 53, 103, 153, . . ., etc.
An STC (Sensitivity Time Control) amplitude threshold circuit 22 operating under the control of the STC function generator 24, serves to compare the amplitude of the detected and unfolded data with a threshold which varies over a portion of the range of the receiver as a decaying function of an R.sup.4 power curve. The range of the threshold extends to a point where discrete sidelobe return signals are predicted as being no longer effective to be mistaken as true target return signals and will be removed by the CFAR threshold circuit. The STC threshold, is also chosen with respect to the predicted amplitude of true target return signals received through the main beam. Therefore, the STC threshold operates on a prediction that over a certain range, true target return signals through the main beam will exceed the threshold and discrete sidelobe return signals will not exceed the threshold. Therefore, for example, if the detected data occurring in a range gate of a first PRF is a true target return signal, then its amplitude will exceed the STC threshold in the range gate which corresponds to the true range of the target. For further understanding of this prior art post detection-STC system, reference is made to FIG. 3, which shows a plot of the STC threshold over a predetermined number of range cells, compared with unfolded data for three PRF's having illustrative range cells of 5 (PRF.sub.5), 6 (PRF.sub.6) and 7 (PRF.sub.7) between respective transmitted pulses. As can be seen from PRF.sub.5, a true target return signal occurred in unambiguous range cells 3, 8, 13, 18, 23, 28, . . ., etc. In PRF.sub.6, a true target return signal occurred in unambiguous range cells 6, 12, 18, 24, 30, . . ., etc. In PRF.sub.7, a true target return signal occurred in unambiguous range cells 4, 11, 18, 25 . . ., etc. As each of the PRF's are compared with the STC threshold, only the target signals having sufficient amplitude to exceed the STC threshold, are passed with an amplitude of "1" to indicate detection. Each of the signals passed by the STC threshold circuit 22, are then received by the range resolver delay line 26 (typically a shift register). This delay line has a 1 amplitude bit (i.e., a "0" for no detection and a "1" for a detection above the STC threshold), by "n" range cells, where "n" is the total display range, by "q" sections, where "q" is the number of PRF's used in the system. Usually, 3 or more PRF's are used. Referring to FIG. 3, PRF.sub.5 is shown as having true target return signals occurring in unambiguous range cells 13, 18, 23 and 28, which exceed the STC threshold. Those range cells will be occupied by "1"'s in the corresponding positions in the range resolver delay line 26. True target return signals which occur in range cells 3 and 8 of the PRF.sub.5 train of signals from the range unfolder 29 are not received at the range resolver delay line 26 since their amplitudes were less than the STC threshold. Similarly, PRF.sub.6 and PRF.sub.7 each supply data to the range resolver delay line 26 and target return signals having amplitudes which exceed the STC threshold are indicated as "1" and occupy corresponding positions in the range resolver delay line. The correlator logic 28 receives the information stored in the range resolver delay line 26 and determines "p" out of "q" correlations in the corresponding range cell positions. Typically, "p" is two or more, but in the examples shown in FIG. 3 and for illustration, p and q are both equal to 3. In the sense used here, a correlation consists of having a "1" in the same unambiguous range cell number in at least p of the q PRF's. Simple logic gates, for example, are used to perform this operation. As shown in FIG. 3, a correlation exists in range cell 18 for each of the PRF's compared. Such a correlation indicates that a true target return signal has been received and the true range corresponds to the 18th range cell.
Problems exist in the prior art system shown in FIGS. 2 and 3, since discrete sidelobe return signals occurring in ambiguous range cells will exceed the STC threshold when the data is unfolded into unambiguous range gates and appear as false targets beyond the true range of the discrete targets. Thus, in FIG. 3, correlation outputs occur in unambiguous range cells 12, 23 and 30 even though the true range of the two discretes (D.sub.1 and D.sub.2) of this example are gates 2 and 5, respectively. These correlations are due to a detection from one target at one PRF correlating with that of other targets at the other PRF's. As a result, the indicated range can be grossly in error from that of any one target alone. These false targets are called "ghosts" and if not removed or prevented from occurring can result in an unusable display of information.